The invention is related to a guidance system for controlling output variables of single-input single-output or multi-input multi-output processes by controlling their derivative variables in such a way that the control performance obtained for said output variables is improved for the high range of control frequencies, i.e. for the range of small control periods with respect to process dynamics.
Such a guidance system is useful in many diverse fields such as aeronautics, naval, nuclear, automation and, in general, in industry. Examples of processes in which the guidance system can be applied are an aircraft where the pitch angle is controlled by elevator position, and a boat where the roll angle is controlled by fin stabilizers.
It is known that Model Based Advanced Controllers use a process cause-effect relationship or prediction model to predict the process output variables evolution. The reliability of said prediction depends on the value of the prediction model parameters. Some solutions require computing the predictive model before it is used in the control system; this is the case of the so called Model Based Predictive Control techniques.
In order to take into account the time-varying nature of processes, other so called Adaptive solutions generate the predictive model in real time at the same time that this model is used to control the process, in such a way that the changes that may happen in the process dynamics can rapidly be incorporated into the predictive model by means of an adaptation mechanism or by a change in the domain of operation. This is the case of the Adaptive Predictive controllers (U.S. Pat. Nos. 4,197,576 and 4,358,822) and the Adaptive Predictive Expert controllers (U.S. Pat. No. 6,662,058 B1).
In both cases, the value of the predictive model parameters depends on the control period. Thus, the setting of the control period is important to ensure that the value of the predictive model parameters lie in a range appropriate to predict reliably the evolution of the process output variables under control. It is known that the choice of the control period below a certain threshold of time, that may be called “modeling threshold”, which is relatively small with respect to the natural time response of the process output variable under control, makes the value of the predictive model parameters approach the zero value too sharply. This makes these parameters increase their sensitivity to any small identification error in their value and, therefore, the use of said parameters value in the prediction of the evolution of the process output variable and in the computation of the control signal to be applied to the process, may significantly deteriorate the controller performance.
However, the practical application of a control system for a certain process output variable may in many cases require the use of a control period below said modeling threshold and, therefore, the performance of model based advanced controllers, adaptive or non adaptive, may, under said control period, deteriorate significantly. For instance, the appropriate control period to be used in the application of a model based advanced controller for the pitch angle of an aircraft acting on the elevator position, should not be smaller than a tenth of a second, while the control period required must be lower than 5 cents of a second to ensure the integrity of the human pilot.
The guidance system of the present invention allows the use of control periods below the previously mentioned modeling threshold improving for these control periods the control system performance obtained by the direct application of model based advanced controllers.